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Concentrations of BOD and total ammonium have decreased in European rivers in the period 1992 to 2006, corresponding to the general improvement in wastewater treatment (Fig. 1). EEA water quality indicators have up to now presented European and regional overviews and country comparison. However, water quality data at national level may not be relevant or sufficient for some countries, and EEA will in the coming years change its indicators to reflect concentration levels and trends at River Basin District (RBD) level to duly reflect local and regional differences in water quality. See also WISE interactive maps: Mean annual BOD in rivers and Mean annual Total Ammonium in rivers

This indicator is updated by 2012 data reported by countries in autumn 2013. The next update will be based on 2013 and 2014 data to be reported by countries in autumn 2015.

Key messages

Concentrations of BOD and total ammonium have decreased in European rivers in the period 1992 to 2006, corresponding to the general improvement in wastewater treatment (Fig. 1).

EEA water quality indicators have up to now presented European and regional overviews and country comparison. However, water quality data at national level may not be relevant or sufficient for some countries, and EEA will in the coming years change its indicators to reflect concentration levels and trends at River Basin District (RBD) level to duly reflect local and regional differences in water quality.

IntroductionOrganic matter, measured as Biochemical Oxygen Demand (BOD) and ammonium, are key indicators of the oxygen content of water bodies. Concentrations of these determinands normally increase as a result of organic pollution caused by discharges from waste water treatment plants, industrial effluents and agricultural run-off. Severe organic pollution may lead to rapid de-oxygenation of river water, a high concentration of ammonia and the disappearance of fish and aquatic invertebrates.

The most important sources of organic waste load are: household wastewater; industries such as paper industries or food processing industries; and silage effluents and manure from agriculture. Increased industrial and agricultural production, coupled with a greater percentage of the population being connected to sewerage systems, initially resulted in increases in the discharge of organic waste into surface water in most European countries after the 1940s. Over the past 15 to 30 years, however, the biological treatment (secondary treatment) of waste water has increased, and organic discharges have consequently decreased throughout Europe. See also CSI 024: Urban waste water treatment.

Overall trend in BOD and ammonium (Fig. 1-3)Concentrations of BOD and total ammonium have decreased in European rivers in the period 1992 to 2006, corresponding to the general improvement in wastewater treatment. The decrease is due mainly to improved sewage treatment resulting from the implementation of the Urban Wastewater Treatment Directive. The economic recession of the 1990s in central and eastern European countries also contributed to this fall, as there was a decline in heavily polluting manufacturing industries.

In European rivers, the oxygen demanding substances measured as BOD and total ammonium have decreased for 50 % and 60 % respectively from 1992 to 2006 (Figure 1).The decrease of BOD has been quite constant, whereas the decrease of total ammonium was larger in the first half of the period.

The largest decrease of BOD occurred in the western European rivers (Figure 2). From 2001 onwards, their concentrations have fallen below the stable concentrations of the northern European rivers (Finnish rivers, respectively). In the eastern European rivers, the concentrations have decreased for a third and reached the level that western European rivers already had in the middle 1990s. The concentrations in the southern European rivers have decreased for almost a third from 2000 to 2006 and become similar to the eastern European rivers concentrations. In the south-eastern European rivers, the decrease has been the lowest (around 20 % from 2000/2001 to 2006) resulting in the highest present concentrations.

The largest decrease of total ammonium (70 %) occurred in the western and eastern European rivers, due to high decrease before 1998, especially in the eastern European rivers (Figure 2). However, in 2006, the concentrations in the eastern European rivers were still 100 g N/l higher than the ones in the western European rivers, and the concentrations in the western European rivers were still more than 100 mg N/l higher than the ones in the northern European rivers with stable trend. In the south-eastern European rivers, the concentrations have decreased for a half from 2001 to 2006 and have fallen bellow the concentrations of the southern European rivers having the smallest decreae (one fourth from 2000 to 2006).

BOD and ammonium trend per country (Figure 4-5)The largest declines in the level of BOD between 2001 and 2006 are observed in Slovenia, Spain, France, Bulgaria, Romania, Poland and Croatia (Figure 4). The decline is also evident in some central European countries (the Czech Republic, Hungary and Slovakia), Denmark and Italy. Austria, the United Kingdom, Belgium, Finland, Bosnia and Herzegovina, Serbia and the Baltic countries (Estonia, Lithuania and Latvia) have quite stable concentrations, which are among the lowest, except for Belgium, Lithuania and Serbia. FYR of Macedonia (FYROM) and Greece have fluctuating values. FYR of Macedonia (FYROM) also faces the highest concentrations. The concentrations have increased only in Albania.

The decline in the level of total ammonium between 2001 and 2006 is evident in south-eastern European countries (except Bosnia and Herzegovina), some central European countries (Slovenia, Poland, Hungary), Italy, the Netherlands and France (Figure 5). Among them, the largest declines are found in FYR of Macedonia (FYROM), Romania, Bulgaria and Albania. Quite stable and low concentrations are observed in the northern European (Norway, Finland, and Sweden) and western European countries (Austria, the United Kingdom, Denmark and Germany), the Baltic countries (Estonia, Lithuania and Latvia) and Bosnia and Herzegovina. Belgium and Spain, the first with the highest concentrations, face high fluctuations. Fluctuating concentrations are also evident in Greek rivers. Increase in total ammonium concentrations has occurred only in Luxembourg.

Stations with significant trend in BOD and ammonium concentration (Figure 6-7)BOD and total ammonium concentrations have decreased at 39 % and 38 % of all stations on the European rivers between 1992 and 2006. Increasing trends in BOD and total ammonium have occurred only on 3 % and 2 % of all stations over the same period.

Most countries have decreasing BOD concentrations at more than 30 % of the stations. Countries having more than 39 % of the stations with significant downward trend come from western (5 countries), eastern (3 countries) and southern Europe (Spain). Luxembourg and Austria have the highest proportion of stations with downward trends in BOD (above 50 %).

In 40 % of the countries (8 out of 20 countries) there are no stations with significant upward trend in BOD concentrations (Fig. 6). In addition, half of the countries (6 out of 12) with significant upward trend face increasing BOD concentrations at less than 3 % of all stations. Only in Macedonia (FYROM), Latvia and Estonia, the proportion of stations with increasing BOD concentrations is more than 10 %.

Similar 43 % of the countries (9 out of 21 countries) have no stations with upward trend in total ammonium concentrations (Fig. 7). Only in Albania, Estonia and Macedonia (FYROM), the proportion of stations with increasing total ammonium concentrations is higher than 5 %, but less than 10 %. Countries having more than 38 % of the stations with significant downward trend come from western and eastern Europe (both 5 countries). Lithuania, Germany and Slovenia have the highest proportion of stations with downward trends in total ammonium (above 60 %).

Slovenia, Ireland, Austria, Latvia and Spain have more than 60 % of all stations with the lowest BOD concentrations (class 1) (Fig. 8). Countries that have fifth or more of the stations with the highest BOD concentrations (class 5) are some south-eastern (FYR of Macedonia (FYROM), Bulgaria and Romania), central European (Hungary, Poland and the Czech Republic) and Benelux countries (Luxembourg and Belgium).

The Northern European (Norway, Finland and Sweden) and some western European countries (Lichtenstein, Ireland, Austria) have more than half of all stations with the lowest total ammonium concentrations (class 1) (Fig. 9). Countries that have more than fifth of the stations with the highest total ammonium concentrations (class 5) are Benelux countries (Belgium, the Netherlands, Luxembourg) and Poland, Bulgaria, Greece and Macedonia (FYROM).

Indicator specification and metadata

Indicator definition

This indicator illustrates the current situation and trends regarding biochemical oxygen demand (BOD) and concentrations of total ammonium (NH4) in rivers. The key indicator for the oxygenation status of water bodies is BOD, which is the demand for oxygen resulting from organisms in water that consume oxidisable organic matter.

Units

Annual average BOD after 5 or 7 days incubation (BOD5/BOD7) is expressed in mg O2/l and annual average total ammonium concentrations in micrograms N/l.

Rationale

Justification for indicator selection

Large quantities of organic matter (microbes and decaying organic waste) can result in the reduced chemical and biological quality of river water, impaired biodiversity of aquatic communities, and microbiological contamination that can affect the quality of drinking and bathing water. Sources of organic matter are discharges from wastewater treatment plants, industrial effluents and agricultural runoff. Organic pollution leads to higher rates of metabolic processes that demand oxygen. This could result in the development of water zones without oxygen (anaerobic conditions). The transformation of nitrogen to reduced forms under anaerobic conditions, in turn, leads to increased concentrations of ammonium, which is toxic to aquatic life above certain concentrations, depending on water temperature, salinity and pH.

Policy context and targets

Context description

There are a number of EU directives that aim to improve water quality and reduce the loads and impacts of organic matter. First, the Water Framework Directive requires the achievement of good ecological status or good ecological potential of rivers across the EU by 2015 and repeals, step-by-step, several older water related directives. Alongside this, the following directives stay in place: the Nitrates Directive (91/676/EEC), aimed at reducing nitrate and organic matter pollution from agricultural land, the Urban Waste Water Treatment Directive (91/271/EEC), aimed at reducing pollution from sewage treatment works and certain industries (see also CSI24 Urban waste water treatment) and the Integrated Pollution Prevention and Control Directive (96/61/EEC) aimed at controlling and preventing the pollution of water by industry.

Targets

The indicator is not directly related to a specific policy target but shows the efficiency of wastewater treatment (see CSI024). The environmental quality of surface waters with respect to organic pollution and ammonium and the reduction of the loads and impacts of these pollutants are, however, objectives of several directives, including the Surface Water for Drinking Directive (75/440/EEC), which sets standards for the BOD and ammonium content of drinking water, as well as other directives mentioned in the previous chapter.

Water Framework Directive (WFD) 2000/60/EC: Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 establishing a framework for Community action in the field of water policy.

Methodology

Methodology for indicator calculation

Data source: Data on rivers is collected annually through the WISE-SoE data collection process. WISE SoE was previously known as EUROWATERNET (EWN) and EIONET-Water. Biological quality elements in rivers have been integrated into the reporting of river water quality, starting from the 2012 reporting period. A formal request is sent to NFPs and NRCs every year with reference to templates to use and guidelines.

The data requested on rivers includes the physical characteristics of the river monitoring stations, proxy pressures on the upstream catchment areas, as well as chemical quality data on nutrients and organic matter, and hazardous substances in rivers. It also includes the biological data (primarily calculated as national Ecological Quality Ratios), as well as information on the national classification systems for each Biological Quality Element and waterbody type. This reporting obligation is an EIONET Priority Data flow.

Station selection: No criteria are used for station selection (except for time series and trend analysis; see below)

Determinants: The determinants selected for the indicator and extracted from Waterbase are BOD5, BOD7, total ammonium and ammonium.

Most countries monitor BOD5. Finland monitors BOD7. Lithuania monitored BOD5 up to 1995 and started monitoring BOD7 in 1996. Latvia monitored BOD7 from 1996 to 2001. Estonia monitored BOD5 in 2010, while it monitored BOD7 up to 2009. BOD is commonly used for BOD5. For countries reporting BOD7, these values have been converted to BOD5 (BOD7 = 1.16 BOD5) for reasons of comparability.

All countries reported total ammonium until 2006. In 2007, Greece and Liechtenstein started reporting ammonium instead of total ammonium. Instead of total ammonium, Cyprus, Lichtenstein and Slovenia began reporting ammonium in 2008, Austria and Netherlands in 2009, Bulgaria and Latvia in 2010, and Estonia, Norway and Poland in 2011. Besides total ammonium, Slovakia also started to report ammonium for some stations in 2008. Belgium, Germany, Italy, Luxembourg, Slovakia and the United Kingdom report either ammonium or total ammonium for an individual station in a selected year from 2008 on. Data of either of the two determinants was included in the assessment. For those stations in Slovakia where both were reported, total ammonium data was included in the assessment.

All values are labeled as BOD5/total ammonium in the graphs, but it is indicated in the graph notes for which countries BOD7/ammonium data are used.

An automatic QA/QC procedure excludes data (stations*year) from further analysis. This is based on flagging in Waterbase, deriving from QA/QC tests. In addition a semi-manual QA procedure is applied, to identify outliers that are not identified in the QA/QC tests. This comprises e.g. values deviating strongly from the whole time series, values not so different from values in other parts of the time series, but deviating strongly from the values closest in time, consecutive values deviating strongly from the rest of the time series or whole data series deviating strongly in level compared to other data series in the country. If not explicitly confirmed valid by reporting countries, such values are flagged in Waterbase, but only excluded from the following year’s assessment due to timing issues. More details on the QA/QC procedure can be found here:

groundwater QA/QC description

rivers QA/QC description

lakes QA/QC description

Quality checked data: In the table on nutrients ("Waterbase_rivers_v12_Nutrients"), QA-fields are treated as follows:

Field "QA_MVissues": all flagged values are excluded from the indicator calculation, except for zero values (flag 103).

Field "QA_outlier": all flagged values are excluded from the indicator calculation, except for outliers confirmed by country (flags 491, 493).

Field "QA_station_issues: all flagged values are allowed (including wrong coordinates or missing coordinates), except for "Water Category value is incompatible with this particular dataset” (flag 511) and “station is not defined in the station table" (flag 599).

Field "QA_CR violation": all flagged values are allowed.

Mean: Annual mean concentrations are used in the time series and present concentration graphics. Countries are asked to substitute any sample results below the limit of detection/determination by a value equivalent to half of the limit of detection/determination before calculating the station annual mean values. Mean concentration values of zero are included in the indicator calculation as zero (0).

Inter/extrapolation and consistent time series

For time series (Fig. 1-5) and trend analyses, only series that are complete after inter/extrapolation (i.e. no missing values in the station data series) are used. This is to ensure that the aggregated data series are consistent, i.e. including the same stations throughout the time series. In this way assessments are based on actual changes in concentration, and not changes in the number of stations.

Changes in methodology: Station selection and inter/extrapolation.

Until 2006, only complete time series (values for all years from 1992 to 2004) were included in the assessment. However, a large proportion of the stations was excluded by this criterion. To allow the use of a considerably larger part of the available data, in 2007 (i.e. when analysing data up until 2005), it was decided to include all time series with at least seven years of data. This was a trade-off between the need for statistical rigidity and the need to include as much data as possible in the assessment. However, the shorter series included might represent different parts of the whole time interval, and the overall picture may therefore not be reliable. In 2009, it was decided rather to inter/extrapolate all gaps of missing values of 1-2 year for each station. At the beginning or end of the data series one missing value was replaced by the first or last value of the original data series, respectively. In the middle of the data series, missing values were replaced by the values next to them for gaps of two years and by the average of the two neighbouring values for gaps of one year.

In 2010 this approach was modified, allowing for gaps of up to three years, both at the ends and in the middle of the data series. At the beginning or end of the data series up to three years of missing values are replaced by the first or last value of the original data series, respectively. In the middle of the data series, missing values are replaced by the values next to them, except for gaps of one year and for the middle year in gaps of three years, where missing values are replaced by the average of the two neighbouring values. Only time series with no missing years for the whole period 1992-2011 after such inter/extrapolation are included in the assessment. The number of gaps is unlimited, only gap length (size) of three years is defined. This procedure increases the number of stations that can be included in the time series/trend analysis. Still, the number of stations is markedly reduced compared to the analysis of the present situation, where all available data can be used. In Figure 1, the two time series are used: 1992–2012 and 2000–2012.

Aggregation of time series

The selected time series (see above) must be aggregated in to a smaller number of groups and averaged, before the aggregated series can be displayed in a time series plot. Determinants are grouped into five geographical regions of Europe, which contain the following countries:

Not all countries listed per region are included in the figures due to no data being reported or no stations with complete time series after inter/extrapolation. Due to changes in the monitoring network (adapting to monitoring networks under Water Directives) the time series are broken and limited number of time series is available for some countries.

Determinants are in addition grouped into six sea region catchments, which are defined not by countries but by river basin districts or river basin district subunits if consistent with catchment areas of seas. The data thus represents rivers or river basins draining into that particular sea. The sea regions are defined as Arctic Ocean, Greater North Sea, Celtic Seas, Bay of Biscay and the Iberian Coast, Baltic Sea, Black Sea and Mediterranean Sea. The sea region delineation is according to the Marine Strategy Framework Directive (MSFD) Article 4, with the Arctic Ocean added as a separate region. As the catchment area draining into what is defined as the North-east Atlantic Ocean region of the MSFD is very big, it was decided rather to use the sub-region level here, but merging the Celtic Seas and the Bay of Biscay and the Iberian Coast.

Determinants are also aggregated for the whole of Europe.

Trend analyses

Trends are analysed by the Mann-Kendall method (McLeod 2005) in the free software R (R Development Core Team 2006). The test was suggested by Mann (1945) and has been extensively used with environmental time series (Hipel and McLeod, 2005). Mann-Kendall is a test for monotonic trend in a time series y(x), which in this analysis is nutrient concentration (y) as a function of year (x). The test is based on Kendall's rank correlation, which measures the strength of monotonic association between the vectors x and y. In the case of no ties in the x and y variables, Kendall's rank correlation coefficient, tau, may be expressed as tau=S/D where S = sum_{i<j} (sign(x[j]-x[i])*sign(y[j]-y[i])) and D = n(n-1)/2. S is called the score and D, the denominator, is the maximum possible value of S. The p-value of tau under the null hypothesis of no association is computed by in the case of no ties using an exact algorithm given by Best and Gipps (1974). The tests reported here are two-sided (testing for both increasing and decreasing trends). Data series with p-value < 0.05 are reported as significantly increasing or decreasing ("strong trends"), while data series with p-value >= 0.05 and <0.10 are reported as marginally significant ("weak trends"). Data series with p-value >0.10 have no significant trend. The test is non-parametric which means that the amount of change from year to year is not considered, only the direction of the change.

The size of the change is estimated by calculating the Sen slope (or the Theil or Theil-Sen slope) (Theil 1950; Sen 1968) using the R software. The Sen slope is a non-parametric method where the slope m is determined as the median of all slopes (yj − yi)/(xj − xi) when joining all pairs of observations (xi,yi). Here the slope is calculated as the change per year for each unit (groundwater body/river station/lake station). This is summarised by calculating the average slope (regardless of the significance of the trend) for all units in Europe or a selected region. Multiplying this by the number of years of the time series gives an estimate of the absolute change over time. This can be related to the mean value of the aggregated time series to give a measure of relative change. The Sen slope was introduced for this indicator in 2013.

The Mann-Kendall method or the Sen slope will only reveal monotonic trends, and will not identify changes in the direction of the time series over time. Hence a combination of approaches is used to describe the time series: A visual inspection of the time series, describing whether the general impression is a monotonic trend, no apparent trend, clear shifts in direction of the trend or high variability with no clear direction; an evaluation of significant versus non-significant and decreasing versus increasing monotonic trends using the Mann-Kendall results; an evaluation of the average size of the monotonic trends using the Sen slope results.

Present concentration distributions

The latest year for which there are concentration data for the selected river stations are extracted from Waterbase. The number of stations with annual mean concentrations occurring in the selected concentration bands or classes are then calculated and presented. The allocation of a station to a particular class is based only on the face value concentration and not on the likely statistical distribution around the mean values.

The new/revised class defining values for BOD5 concentrations (mg O2/l): <1.4, 1.4 to 1.99, 2 to 2.99, 3 to 3.99, 4 to 4.99, >5. The two highest classes are merged to >4.

The new/revised class defining values for total ammonium concentrations (mg N/l): <0.04, 0.04 to 0.09, 0.1 to 0.19, 0.2 to 0.39, 0.4 to 0.99, >1. The two highest classes are merged to >0.4.

Uncertainties

Methodology uncertainty

The methodologies used for aggregating and testing trends in concentration are relatively robust and illustrate the overall European and regional trends. However, uncertainty may be included in evaluating single countries or river basins.

Data sets uncertainty

The data sets for rivers include almost all countries in the EEA area, but the time coverage varies from country to country. The data set provides a general overview of concentration levels and trends of organic matter and ammonia in European rivers. Most countries measure organic matter as BOD over five days but a few countries measure BOD over seven days, which may introduce a small uncertainty in comparisons between countries.

The river monitoring stations included in the assessment vary yearly due to availability of time series for the whole period starting from 1992. In the 2013 assessment, data for a significant number of stations was not reported. Conversely, some new stations were added, if the QA/QC procedure showed that stations reported under different names or codes could be treated as identical. This optimisation needs further quality checking. In the end, 702 stations were assessed in 2013 (compared to 849 stations in 2012) for BOD5 and 921 stations were assessed in 2013 (compared to 952 stations in 2012) for total ammonium.

Rationale uncertainty

Biochemical oxygen demand and total ammonium are well suited for illustrating water pollution with oxygen consumption. However, using annual average values may not fully illustrate the severity of low oxygen conditions.